1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine.
2. Description of the Prior Art
The applicant of the present application has proposed an air-fuel ratio control system having an exhaust gas sensor for detecting the concentration of a certain component of an exhaust gas that has passed through a catalytic converter such as a three-way catalytic converter disposed in the exhaust passage of an internal combustion engine, such as an O.sub.2 sensor for detecting the concentration of oxygen in the exhaust gas, the exhaust gas sensor being disposed downstream of the catalytic converter. The system controls the air-fuel ratio of the internal combustion engine, more accurately, the air-fuel ratio of an air-fuel mixture to be combusted by the internal combustion engine, in order to converge an output of the O.sub.2 sensor, i.e., the detected value of the oxygen concentration, to a predetermined target value for enabling the catalytic converter to have a desired purifying ability irrespective of the aging of the catalytic converter. See U.S. patent application Ser. No. 09/153300, for example.
According to the disclosed technology, a manipulated variable for manipulating the air-fuel ratio of the internal combustion engine, specifically, a target air-fuel ratio or a quantity defining such a target air-fuel ratio, is successively generated in given control cycles in order to converge the output of the O.sub.2 sensor to its target value based on a feedback control process. An exhaust gas sensor (hereinafter referred to as an "air-fuel ratio sensor) for detecting the air-fuel ratio of the exhaust gas that enters the catalytic converter, specifically, the air-fuel ratio of the air-fuel mixture that has been burned by the internal combustion engine, is disposed upstream of the catalytic converter. The amount of fuel supplied to the internal combustion engine is regulated so as to converge the output of the air-fuel ratio sensor, i.e., the detected value of the air-fuel ratio, to a target air-fuel ratio defined by the manipulated variable for thereby controlling the air-fuel ratio of the internal combustion engine at the target air-fuel ratio.
Such air-fuel ratio control for the internal combustion engine is capable of converging the output of the O.sub.2 sensor disposed downstream of the catalytic converter to its target value for thereby enabling the catalytic converter to have a desired purifying ability.
In the above air-fuel ratio control system, the O.sub.2 sensor is used as the exhaust gas sensor disposed downstream of the catalytic converter. However, the exhaust gas sensor may comprise an NOx sensor, a CO sensor, an HC sensor, or another exhaust gas sensor. It is possible to enable the catalytic converter to have a desired purifying ability by controlling the air-fuel ratio of the internal combustion engine so as to converge the output of such an exhaust gas sensor to a suitable target value.
In order to increase the stability and reliability of the control process for converging the output of the O.sub.2 sensor to its target value, a sliding mode control process (more specifically, an adaptive sliding mode control process), which is one type of feedback control process that is highly stable against disturbances, is used to generate the manipulated variable for converging the output of the O.sub.2 sensor to its target value.
The sliding mode control process requires that an object to be controlled be modeled. According to the above technology, it is assumed that the output of the air-fuel sensor is feedback-controlled at a target air-fuel ratio determined by a manipulated variable. Therefore, the object to be controlled by the sliding mode control process is regarded as an exhaust system extending from the air-fuel ratio sensor to the O.sub.2 sensor and including the catalytic converter, and the exhaust system is modeled by a discrete-time system. In order to compensate for the effect of behavioral changes of the modeled exhaust system, there is provided an identifier for identifying, successively on a real-time basis, parameters of the model to be established, using data of the output from the air-fuel ratio sensor and data of the output from the O.sub.2 sensor. According to the sliding mode control process, the manipulated variable is generated by an algorithm constructed on the basis of the model using the data of the output from the O.sub.2 sensor and the parameters of the model identified by the identifier.
Because it is assumed according to the above technology that the output of the air-fuel sensor is feedback-controlled at a target air-fuel ratio determined by a manipulated variable, if the air-fuel ratio sensor fails to operate for some reason, then the air-fuel ratio of the internal combustion engine cannot be appropriately manipulated into the target air-fuel ratio. In such a case, the output from the O.sub.2 sensor positioned downstream of the catalytic converter cannot be controlled at the target value, making it impossible for the catalytic converter to have a desired purifying ability.
One solution is to regulate the amount of fuel supplied to the internal combustion engine according to a feed-forward control process using a map or the like depending on the target air-fuel ratio determined by the manipulated variable which is generated according to the sliding mode control process. At this time, the data of the target air-fuel ratio determined by the manipulated variable may be used instead of the data of the output from the air-fuel ratio sensor for identifying the parameters of the model.
According to the above technology, however, since the model as a basis for generating the manipulated variable is the model of the exhaust system extending from the air-fuel ratio sensor to the O.sub.2 sensor and including the catalytic converter, the model does not take into account behavioral characteristics of the internal combustion engine and their changes. Consequently, even if the manipulated variable for the air-fuel ratio is generated according to the sliding mode control process constructed on the basis of the model, it is difficult make the generated manipulated variable suitable for behavioral states of the internal combustion engine. Even when the air-fuel ratio of the internal combustion engine is manipulated based on the manipulated variable according to the feed-forward control process, it is difficult to manipulate the air-fuel ratio of the internal combustion engine into an appropriate air-fuel ratio required to converge the output of the O.sub.2 sensor to its target value in various behavioral states of the internal combustion engine. As a result, the control process for converging the output of the O.sub.2 sensor to its target value cannot appropriately be performed stably, and hence the catalytic converter fails to have a desired purifying ability.
Furthermore, the above technology is disadvantageous as to cost because the air-fuel sensor is needed in the control process for converging the output of the O.sub.2 sensor to its target value.